Novolac-type phenol resin is obtained by causing reaction of phenols and aldehydes with organic or inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, phosphorous acid, oxalic acid, and p-toluenesulfonic acid. The molecular weight of novolac-type phenol resin is commonly adjusted by the loading ratio or the like of the phenols and aldehydes. Novolac-type phenol resin of low molecular weight tends to have a wide molecular weight distribution. As common means for narrowing molecular weight distribution, there is a method which causes reaction in organic solvent, and a method which removes low molecular weight components by steam distillation or solvent washing. However, novolac-type phenol resin of low molecular weight is not obtained in the former method, and yield greatly diminishes in the latter method.
In response to this problem, for example, a method of manufacture of novolac-type phenol resin has been disclosed wherein phenols and aldehydes are reacted using organic phosphonic acid as the reaction catalyst (see, e.g., Patent Document 1). However, further improvement is desirable with respect to reduction of unreacted phenols, increased yield of novolac-type phenol resin, and so on.